With an increase in the transmission rate of an optical communication system, intensive studies have been conducted on a communication system capable of effective high-capacity high-speed communication. Particularly, DP-QPSK (Dual-polarization Quadra phase shift keying) is most likely to be employed in a 100GE (100 Gigabit Ethernet (Ethernet: registered trademark) transmission device.
In 100GE, a DP-QPSK signal is received by a receiver, and an optical signal contained in the received DP-QPSK signal is separated by polarization and phase. Then, each of the optical signals separated by polarization and phase is converted from optical to electrical signals. Further, the electrical signals generated by opto-electric conversion are A-D converted to obtain digital signals. The receiver that receives such a DP-QPSK signal is already proposed (Patent Literature 1).
In the polarization beam splitting by the receiver described above, the DP-QPSK signal is split by polarization into a TE signal and a TM signal. Accordingly, a device and an element to perform polarization beam splitting are required. As an element to perform polarization beam splitting, an element that splits light entering a waveguide optical device by polarization using the birefringence phenomenon is proposed, for example (Patent Literature 2).
Currently, in 100GE, discussions on various types of MSA (Multi-Source Agreement) are taking place. For example, to make the receiving module compliant with MSA, incorporation of a polarization beam splitting element, a 90° hybrid interferometer, a PD (Photo Diode) and a TIA (Trans Impedance Amplifier) into a 75 mm×35 mm housing is under consideration.
There are several methods for incorporating a polarization beam splitting element for polarization beam splitting, such as integrating a polarization beam splitting element onto an optical circuit board, incorporating a polarization beam splitting element into a module using micro-optics technology, and placing a polarization beam splitting element outside a module.
Further, a method that inserts a polarization beam splitting film into an optical waveguide is proposed (Non Patent Literature 1). FIG. 7 is a diagram showing positions of an optical waveguide and a polarization beam splitting film in the case of carrying out polarization beam splitting by inserting the polarization beam splitting film into the optical waveguide. An optical waveguide 701 is cut at the position where a polarization beam splitting film 702 is inserted. At the position where the optical waveguide 701 is cut, the polarization beam splitting film 702 is inserted.
The reflection characteristics and the transmission characteristics of the polarization beam splitting film 702 vary depending on a difference in the polarization state of incident light 704. Specifically, the polarization beam splitting film 702 transmits a TE component 706 of the incident light 704 and reflects a TM component 705. As a result, the TE component 706 of the incident light 704 propagates through the optical waveguide 701 as it is. On the other hand, the TM component 705 of the incident light 704 is reflected and propagates through an optical waveguide 703. The optical waveguide 701 is thereby split by polarization into the TE component 706 and the TM component 705.